An unlikely duo, of course, since Slackware is a distribution for more advanced Linux users, while Ubuntu is more of a newcomer-oriented distribution. The interesting thing about these two releases is though - they're more alike than you might think, because this Ubuntu release includes, for the first time, the Unity desktop environment as a default. As such, I personally wouldn't recommend this release - which is likely to contain many Unity-related bugs - to newcomers. At least, not when it's running Unity.
Let's focus on Ubuntu 11.04, first. So, it comes with Unity, the brand new desktop environment which replaces GNOME. Users who don't like Unity (I'm one of them) can still opt to use plain-old GNOME in this release, but as you may expect, it's not GNOME 3.0. It's called 'Classic', and you can select it from the login screen. I'd say this is a good solution for now, but personally, I'm still not entirely convinced by Unity at this point.
Unity is of course the big focus of this Ubuntu release, but there's more in there than just this new face. It of course ships the new Firefox 4, and also replaced OpenOffice.org with LibreOffice. The Ubuntu Software Center has also received some love, and during the RC period, I found it very pleasant to use. I'm also digging the fancy new website - easily one of the most accessible Linux distribution websites out there.
As usual, all the Ubuntu derivatives have also seen new releases, with Kubuntu being the most important one.
Moving on to Slackware 13.37 (I see what u did thar), it's all not as disruptive as the Ubuntu release (which shouldn't come as a surprise to anyone. Patrick Volkdering himself explains the most important new features and changes in this release.
"The long-awaited Firefox 4.0 web browser is included, the X Window System has been upgraded (and includes the open source nouveau driver for nVidia cards)," he details, "The venerable Slackware installer has been improved as well, with support for installing to btrfs (for those who would like to try a new copy on write filesystem), a one-package-per-line display mode option, and alienBOB's big surprise: an easy to set up PXE install server that runs right off the DVD!"
Take your pick I suppose!